1. Field of the Invention
The present invention relates to an elastic wave device which utilizes an elastic wave propagating in a piezoelectric body. More particularly, the present invention relates to an elastic wave device including a medium layer disposed between a supporting substrate and a piezoelectric body, in which vibration of the main propagation mode of an elastic wave propagating in the piezoelectric body is distributed to the medium layer.
2. Description of the Related Art
Elastic wave devices have been widely used as resonators and band filters. In recent years, there has been a strong demand for increasing the frequency of elastic wave devices.
Japanese Unexamined Patent Application Publication No. 2004-282232 discloses a surface acoustic wave device which meets the demand as described above. In Japanese Unexamined Patent Application Publication No. 2004-282232, a hard dielectric layer is disposed on a dielectric substrate. A piezoelectric thin film is stacked on the hard dielectric layer. Furthermore, an electrode for exciting a leaky surface acoustic wave is provided on the piezoelectric thin film. In this surface acoustic wave device, the normalized film thickness khp (=2πTp/λ) is in a range of about 0.5 to 1.5, where λ is the wavelength of the leaky surface acoustic wave, and Tp is the thickness of the piezoelectric thin film. It is described that, thereby, the frequency of the surface acoustic wave device can be increased.
In the surface acoustic wave device described in Japanese Unexamined Patent Application Publication No. 2004-282232, an increase in acoustic velocity is achieved by disposing the hard dielectric layer between the dielectric substrate and the piezoelectric thin film. However, in such a structure, not only the leaky surface acoustic wave intended to be used, but also a higher-order mode of the surface acoustic wave propagates while being reflected between the surface of the piezoelectric thin film and the interface between the piezoelectric thin film and the hard dielectric layer. Accordingly, spurious responses due to the higher-order mode occur, thus degrading characteristics, which is a problem.
Furthermore, in the known surface acoustic wave device, when the thickness of the piezoelectric thin film varies due to production variations, characteristics of the surface acoustic wave device vary, which is a problem.